In dental firing furnaces, it is important for the temperature and also the so-called temperature profile, that is to say the course of the temperature over the course of time, to be maintained as precisely as possible, since the quality of the produced dental restoration parts, which can include casts, that is to say metal dental restoration parts, and also sintered parts, depends on this to a great extent.
In sintering operations in particular, the sinter conditions are crucial for achieving the desired material properties of the sintered dental material, which, for example, is pressed in a press furnace. These properties include the strength of the restoration, but also the translucence, especially in the case of crowns and ceramic veneers.
However, the temperature profile in the known dental firing furnaces greatly depends on various factors. Accordingly, it has long been known that the interference effects have to be compensated. Thus, for example, DE-A1 2,856,304 discloses a casting appliance for dental casts with a corresponding control device that can compensate for voltage fluctuations of the mains voltage, and thus provides a more uniform heating result.
It has also been known for some time to provide a temperature regulation, which nowadays is implemented in almost every dental firing furnace, and with which extremely precise regulation of the actual temperature to the setpoint temperature is achieved. To do so, it is necessary to compensate as far as possible for the stored energy in the thermal insulation and for the changing coil resistance of the heating coil, but also, for example, for the compound introduced into the muffle and for the associated temperature reduction.
In dental laboratories, but also in fairly large dental practices that use dental firing furnaces of this kind, it is desirable to achieve the lowest possible cycle time for production of a dental restoration.
It has been proposed to allow the firing furnace to heat up along a special heating curve which, with a very rapid temperature rise to a so-called overtemperature, initially heats the furnace for a certain time to a temperature that lies considerably above the processing temperature, that is to say the desired temperature at which the restoration part is to be processed. This method can also be referred to as overriding. With correct choice of the parameters (temperature, time), it does not damage the restoration part, since, because of the heat capacity of the muffle, the latter heats more quickly during the holding time of the overtemperature, but without reaching its processing temperature. Shortly before the processing temperature in the muffle is reached, the furnace temperature could be lowered to the processing temperature.
To achieve this, it has also been proposed to establish a so-called holding time during which the furnace is held at the overtemperature, while it is subsequently reduced by definition to the processing temperature.
Unfortunately, the thereby improved cycle time and the associated method have not proven themselves in tests. For various reasons, the ceramic is always damaged, presumably because of the overtemperatures, so that, despite the achieved improvement in the cycle time, furnaces of this kind are generally regarded as being difficult to control and risky.
To achieve an improvement in cycle time, without the furnace in question being able to damage the sinter ceramics, it has further been proposed to use a very high, but very brief heat impulse to achieve at least a certain reduction of the heating phase. However, this can only be done using specially equipped and particularly temperature-resistant furnaces in which it is possible, for example, to achieve a temperature of 1400°, instead of the typical overtemperature of 1150°, without damage to the furnace.
Typically, certain ceramics also have to be fired several times. Thus, it is known for a high-firing ceramic, that is to say a ceramic that is sintered for example in the temperature range between 900° C. and 940° C., to be fired first with a so-called dentine firing and, after cooling, for a so-called glaze firing to be applied thereon whose temperature is 20° C. lower, for example, than the temperature of the dentine firing. In firing procedures of this kind, which are still relatively common, two heating phases are thus needed, and this accordingly increases the significance of the heating phase for the overall quality.